The present invention relates to a semiconductor device incorporating a bipolar integrated circuit and an integrated injection logic (IIL or I.sup.2 L).
The conventional device shown in FIG. 1 is structured for its IIL section 200 to have an n.sup.+ -type buried layer 21 made of impurity with a high diffusivity in order to improve the current gain and operating speed of the IIL and enhance the breakdown voltage of the bipolar transistor, resulting in an upward-diffusion structure of the n.sup.+ -type buried layer 21 expanding into an n-type epitaxial layer 11 to a much greater extent than the buried layer 2 for a bipolar transistor section 100, as described in Japanese Patent Publication No. 55-13585. In fabricating the above-mentioned structure, buried layers 2 and 21 and an epitaxial growth layer 11 are formed on a substrate 1. p-type impurity regions 3 are formed to isolate the component regions, in which p-type impurity regions 20, 201 and 2011 are formed. Thereafter, n-type high-impurity concentration regions 30 and 301 are formed in the p-type impurity regions 20 and 2011 and in the n-type epitaxial growth layer 11. Finally, electrodes 5, 6, 7, 8, 9 and 10 are formed on the respective regions.
In such a prior art arrangement, the buried layer 21 has its upper and lower diffusing portions having an extremely low impurity concentration. Accordingly, a high-impurity concentration n.sup.+ -type buried layer 211 in the IIL section 200 has a depth D comparable with the depth d of the buried layer 2 in the bipolar transistor section 100, and there is no significant difference in the Gummel numbers (which represents the integrated value of impurity concentration) of the n.sup.+ -type buried layers between the bipolar section and the IIL section. On this account, no special consideration has been made in this prior art device structure regarding the problem that a current flowing through the buried layer to the substrate increases as the depth of the n.sup.+ -type buried layer is decreased for minimizing the thickness of the component.